The present invention generally relates to a cardiac monitor. The present invention more particularly relates to a fully implantable cardiac monitor for monitoring the physiology of the heart and which is externally programmable for detecting either arrhythmias of the heart or ischemia or both. The implantable cardiac monitor generates data indicative of these conditions and stores the data in memory for later retrieval externally of the patient through telemetry. The present invention is further directed to electrode systems for use with the implantable cardiac monitor for sensing heart activity.
Coronary artery circulation normally supplies sufficient blood flow to the heart to meet the demands of the heart muscle (myocardium) as it labors under a widely varying workload. An imbalance that arises between this supply and demand usually precipitates angina pectoris (pain). When the imbalance becomes excessive, myocardial infarction results. Myocardial infarction is necrosis or death of cardiac tissue resulting from the lack of blood flow to the heart. For example, the narrowing of a major coronary artery by more than fifty percent (50%) impairs nutrient blood flow under conditions of myocardial demand.
By far the most common underlying pathologic process that gives rise to the narrowing of a major coronary artery is atherosclerosis. In most patients suffering from atherosclerosis, plaque develops in the proximal segments of the coronary arteries. In other patients, this condition may be diffuse and may occur in both proximal and distal vessels.
Increases in oxygen consumption cause ischemia if coronary artery blood flow cannot rise to meet a higher demand. The clinical manifestations of ischemia are angina, myocardial infarction, congestive heart failure, and electrical instability (arrhythmia). The last mentioned symptom is assumed to account for most of the sudden cardiac death syndrome patients.
Silent ischemia (ischemia without angina) is common and may result in a myocardial infarction without warning. It has been reported that twenty-five percent (25%) of patients hospitalized with a myocardial infarction have no pain and over fifty percent (50%) of ischemic episodes occur without associated pain.
In treating ischemia, the primary goal of medical therapy is to reduce oxygen consumption and increase blood supply by reducing vascular tone (improving collateral flow) preventing thrombosis and opening or bypassing the blockage in the artery or arteries affected. If a clot is causing the blockage, a thrombolytic drug may be used to open the occluded artery. The most direct way to increase blood supply is to revascularize by coronary artery bypass surgery or angioplasty.
Cardiac electrical instability (arrhythmia) may occur during ischemic events and is also a common condition after a myocardial infarction. Since cardiac arrhythmias such as ventricular tachycardia can degenerate into ventricular fibrillation which is life threatening, these arrhythmias are of great concern to the physician or cardiologist. To control such arrhythmias, the cardiologist may choose to treat the patient with antiarrhythmic drugs.
The testing of the effectiveness of such drugs in reducing the number and severity of arrhythmias is very difficult. This is due to the fact that arrhythmias occur at any and all times. In attempting to test the effectiveness of such drugs, patients are often required to wear an external monitor for periods of twenty-four (24) or forty-eight (48) hours that record all cardiac signals during these periods. Also, the physician may submit a patient to extensive electrophysiologic testing which is often performed in a hospital. The physician then uses the results of such testing to assist in determining the course of such drug therapy.
Myocardial infarctions often leave patients with a permanent arrhythmogenic condition even after coronary artery bypass surgery or angioplasty. Such patients are in need of close monitoring for both arrhythmic events as well as further deterioration of the patency of the cardiac vessels.
After revascularization to increase blood supply to the myocardium, the cardiologist must continually submit such patients to diagnostic tests to determine if the revascularization procedure has remained effective. In angioplasty patients, studies have indicated that twenty-five percent (25%) of those patients will experience restenosis within a period of six (6) months. In those patients having coronary artery bypass surgery, restenosis may occur anywhere from a few hours to several years from the time of such surgery. Studies have indicated that after approximately five (5) years, patients having coronary artery bypass surgery should be monitored closely.
To diagnose and measure ischemic events suffered by a patient, the cardiologist has several tools from which to choose. Such tools include twelve-lead electrocardiograms, exercise stress electrocardiograms, Holter monitoring, radioisotope imaging, coronary angiography, myocardial biopsy, and blood serum enzyme tests. Of these, the twelve-lead electrocardiogram (ECG) is generally the first procedure to be utilized to detect a myocardial infarction. An exercise stress electrocardiogram is generally the first test to be utilized for detecting ischemia since resting twelve-lead electrocardiograms often miss the symptoms of ischemia. Unfortunately, none of the foregoing procedures provide an ongoing and continuous evaluation of a patient's condition and are therefore only partially successful at providing the cardiologist with the information that the cardiologist requires in determining the proper corrective course of action.
There is therefore a need in the art for an implantable cardiac patient monitor capable of providing twenty-four (24) hour a day monitoring of patients for either the sudden onset of restenosis or a new occlusion or a serious arrhythmic event. The present invention provides such an implantable cardiac patient monitor. By virtue of the present invention, long-term trends in ischemia, heart rates and arrhythmias may be monitored and recorded. Also, by virtue of the present invention, high-risk patients can be instructed to seek aid immediately to avoid permanent cardiac tissue damage due to a thrombus. In addition, by virtue of the present invention, the cardiologist can use the ischemia trend data to guide further therapy to match changing conditions of a patient whether the patient is improving or deteriorating. Arrhythmias common to myocardial infarction patients may also be monitored and these conditions may also be trended as well. Such information can be especially useful to the cardiologist in adjusting antiarrhythmic drug therapy to maximize such therapy and minimize side effects. Hence, the implantable cardiac patient monitor of the present invention is capable of providing the cardiologist with ischemic and heart rhythm information not previously available in the prior art which will enable a physician to eliminate or delay certain diagnostic tests and enable the physician to maximize drug therapy.